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March, 2018
Silver Sintering: A Reliable Alternative to Lead-Free Bonding
By Marco Koelink, Business Development and Commercial Manager, Advanced Packaging Center, and Michiel de Monchy, European Applications Manager Die Attach and Preforms, Alpha Assembly Solutions
T
he challenges of working with lead-free bond- ing materials are now facing stiff competition from silver (Ag) sintering. Ag sintering, or
low-temperature diffusion bonding is gaining popu- larity for its excellent electrical and thermal conduc- tivity. Due to some interesting optical properties, potential applications range from power electronics to printable electronics and (optical) biosensing. Developments in Ag sintering are being driv-
en by the replacement of lead-containing bonding materials and applications in power electronics. Specifically, it can be used for applications that have sensitive energy efficiency requirements, due to limited power availability, such as electric vehi- cles (EVs). Main considerations in transitioning from tra-
ditional bonding materials to Ag sintering are cost and reliability. As early adopters of this technology are pioneering its application on an industrial scale, more and more data will become available. As the technology matures and the number of applications grows, prices are expected to drop and become more competitive with traditional bonding materials.
Silver Sintering Over the last 10 years, many developments in
solder technologies have been driven by interna- tional legislation that requires lead-free solder materials to improve the reliability of the joints. The introduction of electric and hybrid vehicles is spurring the demand for efficient, high-power elec- tronics, mainly to improve their driving range. Several technologies have been introduced to
achieve high-power modules with high reliability. These include gold base, high-cost solders, such as gold-germanium (AuGe) and gold-tin (AuSn), tin- antimony (SnSb) alloys, and Ag sintering. Advanced Packaging Center and Boschman Technologies have
Boschman Sinterstar Innovate-F-XL universal sintering system.
major drivers for this process is the change in free energy within the silver sintering product. Smaller particles will have more free energy and need less external energy to initiate the fusion process. ALPHA® Argomax®, a product group developed by Alpha Assembly Solutions, contains particle agglomerates of about 20 nm in size, allowing sin- tering parameters at temperatures comparable to or lower than those of lead-free solder reflow.
worked together, specifically on dynamic insert technology, to industrialize Ag sintering. Ag sintering is based on solid state diffusion,
where silver particles are fused to each other and to the metallization of dice and substrates. One of the
This temperature, together with the relative-
ly low pressures of 10 MPa, allows a wider range of products to be sintered. Argomax can be delivered in either paste or pre-dried film, due to its special sintering inhibitor. The silver will fuse only if the interface mate-
rials are purely metallic. Also, both the metalliza- tion of the dice and substrates must be relatively free of oxides. The easiest surface to bond to is sil- ver itself. If silver cannot be used, noble materials, such as gold, palladium and platinum are the next best option. The materials do not need to be more than 0.04 mil (1 µm) thick, because the diffusion of the silver only penetrates 25 to 75 nm deep. With another Argomax material, it is possible
to sinter to copper in an ambient atmosphere. Unfortunately, it is not possible to bond to surfaces that have dense oxide structures, such as nickel and aluminum, nor can bare silicon be used. Ag sintering offers a new void-free die attach
technology with high thermal and electrical conduc- tivity (up to 300 W/m-K and 2.5 µWcm). The Ag sin- tering process is defined either by temperature and time or by temperature, time and pressure. While the process defined by temperature and time (pres- sure-less) is relatively easy to industrialize with reflow ovens, etc., the other process requires accu- rate and independent control of all three variables. Boschman has developed both semi-automat-
ed and fully automated equipment, using high-pre- cision dynamic insert pressure control, in combina- tion with sophisticated temperature control. These systems provide automated control of the sintering process with programmable temperatures up to 608°F (320°C), variable pressure between 10 and 30 MPa and a maximum sinter area of 13.8 x 10.6 in. (35 x 27 cm).
Continued on next page
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